Isononanoic acid, a mixture of structurally branched C9 monocarboxylic acids, is an important intermediate in industrial organic chemistry which is processed to give a multitude of conversion products for a wide variety of different fields of use. For example, the salts thereof are used as drying accelerators or siccatives for coatings.
Isononanoic acid is processed further in large volumes to give carboxylic esters which find use as lubricant. Particularly esterification with polyhydric alcohols such as neopentyl glycol, trimethylolpropane, ditrimethylolpropane, pentaerythritol or dipentaerythritol gives lubricants which are used in the operation of refrigerators. Isononanoic acid is frequently esterified in a mixture with other C4-C12-monocarboxylic acids such as 2-methylbutyric acid, n-pentanoic acid, n-heptanoic acid, 2-ethylhexanoic acid or n-octanoic acid (EP 1 281 701 A1, EP 1 199 300 A2, EP 0 903 335 A1, EP 0 475 751 A1, WO 90/12849 A1).
Esters of isononanoic acid are likewise used as plasticizers for thermoplastic polymers. Plasticizers for PVC based on isononanoic acid with polyols are described, for example, in WO 95/19389 A1. A specific class of ester plasticizers, which are also abbreviated to the term G esters, contains ether diols such as diethylene glycol, triethylene glycol or tetraethylene glycol as the alcohol component. They are utilized for plasticization of polyvinyl butyral films which are used as the intermediate layer in the production of multilayer or composite glasses. They can likewise be used as coalescence agents or film-forming aids in aqueous dispersions of polymers which find various uses as coating materials (DE 10 2009 048 771 A1, DE 199 40 991 A1). According to EP 2 308 821 A2, the crude esterification product is treated with ozone and ozone-containing gases to lighten the colour, and immediately thereafter subjected to a steam treatment.
For said uses, predominantly an isononanoic acid containing the structurally isomeric form 3,5,5-trimethylhexanoic acid as the main constituent is used. The C-9 hydrocarbon skeleton 3,5,5-trimethylhexyl is based on the petrochemical precursor isobutene, which is dimerized in the presence of acidic catalysts to give diisobutene and is separated by distillation from the higher oligomers likewise formed (Hydrocarbon Processing, April 1973, pages 171-173; Ullmann's Encyclopedia of Industrial Chemistry, 6th Ed., 2003, Vol. 6, page 3). Diisobutene consists essentially of the isomeric octenes 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene and can be converted to the corresponding aldehyde 3,5,5-trimethylhexanal by the oxo process or hydroformylation reaction with carbon monoxide and hydrogen in the presence of rhodium or cobalt catalysts (Ullmann's Encyclopedia of Industrial Chemistry, 6th Ed., 2003, Vol. 2, page 68, 75; DE 2737633 A). Further C9 isomers present in small amounts are 3,4,4- and 3,4,5-trimethylhexanal, and also 2,5,5-trimethylhexanal, 4,5,5-trimethylhexanal and 6,6-dimethylheptanal. Oxidation of this aldehyde mixture gives an industrially available isononanoic acid typically having a content of 3,5,5-trimethylhexanoic acid of about 90% (Ullmanns Encyklopädie der technischen Chemie, 4th Edition, 1975, Verlag Chemie, Volume 9, pages 143-145; EP 1 854 778 A1). According to DE 199 08 320 A1, dibutene which is separated from an oligomerized butene material is converted through hydrocarboxylation or through hydroformylation with subsequent oxidation to a mixture of isomeric C9 monocarboxylic acids, which is subsequently converted to vinyl esters. These vinyl esters can be used as plasticizers.
The most important raw material source for isobutene is the C4 cut from the steamcracking of naphtha. The availability thereof compared to the C2 and C3 cracking products can be controlled by the conditions of steamcracking and is guided by the market conditions. 1,3-Butadiene is first removed from the C4 cracking products by extraction or by selective hydrogenation to n-butenes. The resulting C4 raffinate, also called raffinate I, comprises predominantly the unsaturated butenes isobutene, 1-butene and 2-butene, and the hydrogenated products n-butane and isobutane. Isobutene is removed from the raffinate I in the next step, and the resulting isobutene-free C4 mixture is referred to as raffinate II.
For the isobutene removal, various processes are employed in industrial production, in which the highest reactivity of the isobutene in relative terms in the raffinate I is exploited. A known method is the reversible proton-catalysed addition of water to give tert-butanol, or methanol addition to give methyl tert-butyl ether. Isobutene can be recovered again from these addition products by redissociation (Weissermel, Arpe, Industrielle Organische Chemie [Industrial Organic Chemisty], VCH Verlagsgesellschaft, 3rd Edition, 1988, p. 74-79).
It is likewise possible to contact the butadiene-free C4 raffinate at elevated temperature and under pressure with an acidic suspended ion exchanger. Isobutene oligomerizes to diisobutene, triisobutene, and in a small portion to higher oligomers. The oligomers are separated from the unreacted C4 compounds. It is then possible to obtain diisobutene or triisobutene in pure form by distillation from the oligomer. The dimerization of n-butenes with isobutene forms the co-dimer to a small degree (Weissermel, Arpe, Industrielle Organische Chemie, VCH Verlagsgesellschaft, 3rd Edition, 1988, p. 77; Hydrocarbon Processing, April 1973, p. 171-173).
Against the background that the availability of octenes based on the C4 cut from naphtha cracking is limited and depends on the local conditions, it is desirable to develop further octene sources based on inexpensively available large-scale products which can be transported to various sites in a simple manner.
2-Ethylhexanol is available inexpensively as an industrial large-scale product which can be sold widely without any problems. As is well known, 2-ethylhexanol is prepared on the industrial scale by hydroformylation or oxo process using propylene to give n-butyraldehyde with subsequent alkali-catalysed aldol condensation to give 2-ethylhexenal followed by full hydrogenation to give 2-ethylhexanol (Ullmann's Encyclopedia of Industrial Chemistry, 7th Edition, 2011, Wiley, Volume 13, pages 579-584).
WO 03/029180 A1 briefly discusses the use of 2-ethylhexanol for preparation of an octene mixture which is processed via dehydration, hydroformylation and hydrogenation to give an isononanol mixture. The emphasis is on the adjustment of the viscosity of the isomeric dialkyl phthalates which are obtained by esterification of isomeric nonanols with phthalic acid or phthalic anhydride. No pointers are given to convert the dehydration products of 2-ethylhexanol to isononanoic acid or isononanoic esters.
The utilization of 2-ethylhexanol as the octene source enables the provision of isononanoic acid based on propylene, and reduces dependence on octene availability based on butene. An isononanoic acid prepared in this way can then be processed further to give carboxylic esters.